This invention relates generally to thermally responsive electrical switches and more particularly to such switches which are useful as protectors for electrical motors such as those used for hermetic compressors.
Electrical motors which operate compressors in refrigerators and air conditioners, and the like, are typically enclosed in shells together with selected coolant fluids. Such motors are normally provided with motor protector devices, sealed to exclude the coolant fluids, fitted on or near the motor windings to optimize their thermal response to heat generated by the windings. A thermally responsive member is arranged within the typical motor protector device to respond to the winding temperature for moving electrical contact means to open a circuit when the thermally responsive member is heated to a selected temperature. Usually a heater is disposed in heat-transfer relation to the thermally responsive member to be connected in series with the motor for promptly heating the thermally responsive member to open the circuit to the motor when an overload current occurs in the motor""s windings. An example of such a motor protector is shown and described in U.S. Pat. No. 4,376,926 assigned to the assignee of the present invention. In motor protectors of this type, a bimetallic, snap-acting disc is cantilever mounted on a heater which in turn is mounted on a terminal pin extending through a header plate electrically isolated from the plate which may be provided with electrically insulated material surrounding the pin to provide a suitable dielectric distance between the pin and the header. An electrically conductive tube shaped housing having a closed end is hermetically attached to the header and is provided with a stationary electrical contact attached to the housing wall in the vicinity of its closed end. A movable electrical contact is mounted on the distal end portion of the disc with the disc adapted to move between a normally contacts engaged, dished shape disc having a concave configuration facing the stationary contact to a contacts opened, convex configuration facing the stationary contact upon being heated to a preselected temperature. The preselected temperature is chosen to prevent overheating of the motor.
In making the above described motor protector, calibration is typically effected by deforming the housing to move the stationary contact to obtain the preselected opening temperature, i.e., the actuation temperature of the disc.
It is an object of the present invention to provide an improved, lower cost yet reliable hermetic motor protector for use with hermetic compressors, such as 240 VAC, 50/60 Hz compressors. Another object is the provision of a motor protector capable of being mounted on or near a motor winding and having up to 90 amps locked rotor capacity. Yet another object of the invention is the provision of a hermetic motor protector for use with compressor motors which require no temperature calibration after assembly of the protector for meeting motor protection requirements.
Briefly, in accordance with the invention, an elongated header plate is formed with first and second pin receiving apertures through which extend glass sealed, respective first and second terminal pins. A stationary contact is attached to the first terminal pin and a resistive heater element formed in the shape of a loop has one end attached to the second terminal pin and a second end attached to the header plate. A bracket is mounted to the header plate and is formed with a disc seat spaced above the header plate. The disc seat is provided with a vertically upstanding tab which is received in an aperture of a disc, to be discussed below. The bracket is also formed with a generally U-shaped portion having a first leg disposed on the header plate and a second leg spaced above the header plate and extending in a direction toward the stationary contact. A bimetallic, dished shaped circular disc having a centrally located aperture therethrough is disposed on the disc seat with the upstanding tab of the disc seat received through the aperture in the disc and with an edge of the disc received under the distal free end of the second leg of the U-shaped bracket portion. A movable spring contact arm, also preferably provided with an aperture, has one end cantilever mounted to the top of the second leg of the U-shaped bracket portion with the aperture in the arm aligned with the upstanding tab of the disc seat. A movable contact is mounted on the bottom side of the spring contact arm and is normally biased into electrical engagement with the stationary contact. The spring contact arm is preferably provided with a downwardly extending motion transfer projection such as by deforming a portion of the spring contact arm into a bump, which is aligned with the edge portion of the disc diametrically opposite the second leg of the U-shaped bracket portion. When the disc snaps from a normal downwardly facing, i.e., toward the header plate, concave configuration at ambient temperature with the disc completely unrestrained to an oppositely shaped downwardly facing convex configuration, the disc moves against the second leg and pivots on the disc seat with the disc performing as a lever so that the outer peripheral portion of the disc which is diametrically opposite to the second leg engages the motion transfer projection and moves the spring contact arm upwardly and concomitantly the movable contact out of electrical engagement with the stationary contact. The top of the disc seat against which the disc pivots as a fulcrum comprises the bight portion of another U-shaped portion of the bracket with the bight positioned within a selected window of vertical distances, relative to the face of the header plate, from the bottom of the motion transfer projection of the spring contact arm. The contacts closed contact force can be adjusted by applying a downward force on the second leg. Assembly of the protector is completed by hermetically welding an elliptical dome shaped housing with a portion of the housing preferably being deformed at a location aligned with the movable contact to serve as a motion limiting stop.